Preserve literal types in the expression resolver (PHPStan-style)

[calebdw]

Summary

The expression resolver (resolve_rhs_expression) currently widens
scalar literals to their base types (string, int, float) at
the point of resolution. This means compound expressions like
ternaries, match, and null-coalesce lose literal precision and require
a separate post-hoc narrowing pass in the diagnostic path
(narrow_literal_type) to recover it.

PHPStan preserves literal types from the start and only widens where
specifically needed. Adopting this approach would make the type system
more precise across the board — diagnostics, hover, completion — and
eliminate the need for the narrowing workaround.

Current state

• resolve_rhs_expression returns PhpType::string() for
  Expression::Literal(Literal::String(...)), etc.
• The diagnostic path uses narrow_literal_type() to recover
  precise literal types for argument type checking.
• Array shape and list element tracking depend on the widened types
  (array{name: string} vs array{name: 'John'}).

What needs to change

1. Make resolve_rhs_expression return literal types for scalar
   literals (PhpType::literal_string_raw, literal_int,
   literal_float).
2. Add a widening step in the array shape and list tracking layers
   so that $arr['name'] = 'John' still infers
   array{name: string} rather than array{name: 'John'}.
3. Audit and update tests — an initial attempt showed 15 failures
   in shape/list tracking tests that expect widened types.
4. Once the resolver preserves literals, remove the
   narrow_literal_type workaround from the diagnostic path.

Context

Discovered while fixing #180. The narrowing approach works for the
immediate issue but is architecturally fragile — it requires
enumerating every compound expression form rather than getting
precision for free from the resolver.